1. Field of the Invention
The present invention relates to an electron gun for a cathode-ray tube and, more particularly, to an electron gun, used for a cathode-ray tube, which can optimize a beam spot on a target.
2. Description of the Related Art
In a cathode-ray tube such as a monochrome picture tube, a color picture tube, and a projection tube, an electron beam emitted from an electron gun is deflected by a magnetic field generated by a deflection yoke mounted around an envelope so as to be horizontally and vertically scanned on a phosphor screen (target) formed on the inner surface of the envelope. With this operation, an image is reproduced on the phosphor screen.
Electron guns of various schemes have been used for such cathode-ray tubes. Every conventional electron gun includes an electron beam forming portion, constituted by a cathode and a plurality of grids (electrodes), for controlling electrons emitted from the cathode and focusing the electrons to form an electron beam, and a main electron lens portion, constituted by a plurality of grids, for focusing the electron beam emerging from the electron beam forming portion onto a phosphor screen.
FIG. 1 shows such an electron gun. This electron gun comprises a cathode K and first to fourth grids G1 to G4 sequentially arranged at predetermined intervals from the cathode K to a phosphor screen 1. The first and second grids G1 and G2 are constituted by plate-like electrodes having relatively small electron beam passage holes formed in their surfaces according to the cathode K. The third and fourth grids G3 and G4 are constituted by cylindrical electrodes having relatively large electron beam passage holes formed in their end faces. In the electron gun, when predetermined potentials are applied to the cathode K and the first to fourth grids G1 to G4, an electron beam forming portion GEA is formed by the cathode K and the first, second, and third grids G1, G2, and G3, and a main electron lens portion MLA is formed by the third and fourth grids G3 and G4.
FIG. 2 shows an optical model of electron lenses formed in the electron gun. Electrons emitted from the cathode K are focused to form a crossover point CO owing to the effect of the cathode K and the first and second grids G1 and G2. The electrons are then slightly focused by a prefocus lens PL formed by the second and third grids G2 and G3 to form an electron beam 2. The electron beam 2 diverges and enters the third grid G3. The electron beam 2 which enters the third grid G3 is subsequently focused onto the phosphor screen 1 by the main electron lens portion constituted by a cylindrical electron lens portion MLA formed by the third and fourth grids G3 and G4.
In order to optimize image characteristics, especially resolution, of an image formed on the phosphor screen upon horizontal and vertical scanning of the electron beam 2, it is required that the size of the beam spot of the electron beam 2 focused on the phosphor screen 1 be minimized. Since a decrease in beam spot size is closely associated with the lens performance of an electron lens, the lens performance must be improved.
Especially the main electron lens portion MLA has conventionally employed several types of electrode arrangements. In order to improve the lens performance, however, the following two types of lenses are basically effective: a large-aperture electron lens obtained by increasing the diameter of an electron beam passage hole in each grid; and a long-focus electron lens which is obtained by increasing the grid pitch to realize small potential changes.
In general, however, since an electron gun for a cathode-ray tube is sealed within a neck constituted by a narrow glass cylinder, the size of an electron beam passage hole formed in each grid is geometrically limited. With this limitation, a corresponding lens aperture is limited. In addition, it is required that a focusing electric field formed between grids be free from the influences of other undesired electric fields formed in the neck. For this purpose, the grid pitch is also restricted. As a result, it is very difficult to improve the lens performance of an electron gun for a cathode-ray tube.
An in-line gun assembly for emitting three electron beams in line, which consists of a center beam and a pair of side beams propagating in the same plane, is widely used especially for a color cathode-ray tube. Since this electron gun assembly for emitting three electron beams in line must be sealed within a neck constituted by a narrow glass cylinder, electron beam passage hole through which the respective electron beams pass are inevitably reduced in size. This makes it more difficult to improve the lens performance.
With regard to an improvement in the lens performance of an electron gun for a color cathode-ray tube, for example, Published Unexamined Japanese Patent Application No. 1-267639 discloses an electron gun having a large-aperture electron lens common to the three electron beams. However, since the large-aperture electron lens of an electron gun disclosed in such an official gazette is a cylindrical lens, the problem of aberration is posed. Since aberration with respect to a pair of side beams is especially large, it is difficult to simultasneously reduce the sizes of the beam spots of the center beam and the pair of side beams on a phosphor screen. In addition, since the structure of such an electron lens is very complicated, a problem is posed in terms of practical applications.
As described above, in order to improve the resolution of a cathode-ray tube, the size of the beam spot of an electron beam focused on a phosphor screen must be minimized. For this purpose, it is required that the lens performance of each electron lens of an electron gun be improved. Especially with regard to a main electron lens portion, it is known to improve lens performance by employing either of the two types of electron lenses: a large-aperture electron lens formed by increasing the diameter of an electron beam passage hole in a grid, and a long-focus electron lens formed by increasing the grid pitch so as to have small potential changes. However, in an electron gun which is sealed within a neck constituted by a narrow glass cylinder, geometrical limitations are inevitably imposed on the lens aperture of such a large-aperture electron lens or on the grid pitch of such a long-focus electron lens. For this reason, it is very difficult to improve the lens performance of an electron gun for a cathode-ray tube. In addition, with regard to an in-line gun assembly for emitting three electron beams in line, which consists of a center beam and a pair of side beams propagating in the same plane, since this gun assembly must be sealed within a neck constituted by a narrow glass cylinder, an electron beam passage hole in each electrode is inevitably reduced in size. This makes it more difficult to improve the lens performance. In some electron gun assembly for a color cathode-ray tube, a large-aperture electron lens common to the three electron beams is formed to improve the lens performance. However, since the large-aperture electron lens of this conventional electron gun assembly is a cylindrical lens, the problem of aberration is posed. Since aberration with respect to a pair of side beams is especially large, it is difficult to simultaneously reduce the sizes of the beam spots of the center beam and the pair of side beams on a phosphor screen. In addition, since the structure of such an electron lens is very complicated, a problem is posed in terms of practical applications.